Optical pickup devices adopting a three-beam method or a push pull method as a tracking servo method are known.
As shown in FIG. 32, in the three-beam method, a main beam S1 is converged on the surface of an optical disk, and sub-beams S2, S3 are converged at both sides of the main beam S1. Reflected light from the disk of the sub-beam S2 and that of the sub-beam S3 are detected by photodetectors D1, D2 respectively. A differential signal D1–D2 between both reflected light is taken as a tracking signal.
In the push pull method, as shown in FIGS. 33(a) and (b), beams L emitted from a laser 31 are converged on the surface of an optical disk 35 by an objective lens 34. Beams reflected from the optical disk 35 are guided to a two-part photodetector 36 by a polarization beam splitter 33. At this time, a differential signal D1′–D2′ of signals from light-receiving regions D′, D2′ of the two-part photodetector 36 is taken as a tracking signal.
In the three-beam method, the interval between the sub-beams S2 and S3 to generate the tracking signal is very large. Therefore in the case where tracking is performed from an information-unrecorded portion of the optical disk to an information-recorded portion thereof, as shown in FIG. 32, there is a big difference between the reflected light of the sub-beam S2 and that of the sub-beam S3. Thus the three-beam method has a problem that a tracking offset is generated in the differential signal D1–D2 of the photodetectors D1, D2 owing to the influence of the difference between the amount of the reflected light of the sub-beam S2 and that of the reflected light of the sub-beam S3 other than a tracking deviation signal.
At the stage of assembling the pickup adopting the three-beam method, it is necessary to accurately make a positional adjustment so that each of the sub-beams S2 and S3 is applied to a predetermined track. Thus three-beam method has a problem that it takes long to assemble the pickup.
In the three-beam method, because three beams are generated from one light source, it is necessary to make the output of the light source large to secure the amount of the main beam which is used to record and reproduce information. Thus the three-beam method has a problem that the light source has a large burden so that the power consumption increases.
The problems which arise in the three-beam method would not occur in the push pull method in principle, but the push pull method has the following problem.
In the push pull method, the difference in the amount of the light reflected from the optical disk 35 between the right side and the left side of the light amount distribution is detected to obtain the tracking signal. If the objective lens moves in a radial direction in this tracking, there may occur a deviation in the optical axis of the light reflected from the optical disk 35, as shown in FIG. 33(a). Therefore the beam center may deviate from the center of the two-part photodetector 36.
When the optical disk 35 has a tilt, as shown in FIG. 33(b), the light reflected from the optical disk 35 returns to the objective lens 34 with the reflected light tilting relative to the objective lens. Therefore the center of the reflected light deviates from the objective lens 34.
As described above, although the tracking itself is correct in both cases, an offset is generated in the differential signal D1′–D2′ of the signals from the light-receiving regions D1′, D2′ of the two-part photodetector 36. Therefore tracking cannot be performed favorably.